The first thing you need to go and get the mail. If you are like me you likely get a piece of junk mail everyday or so. You might get a piece of junk mail and credit card junk mail a day even. The more things you signup to or belong to the higher your chances of getting this junk and the more you will get it's the price you pay for having a life. Now that you got the mail find the junk mail.
The first thing that you need to do is take everything out. When you take out everything look for the return envelope and put that in a safe place away from the rest of the mail for safe keeping for use later.
The next step you will want to take is to tear, shred or in any way you can to remove your information so that no one else will have your private information. This is a safe precaution you should always do with your mail to keep you safe from identity theft (well one thing you can do).
The next step is entirely up to you and depends on how much time you want to waste and how fed up you are with the junk mail, if one particular company sends you a ton you might want to be extra mean at this step. You should take the trash they sent you and place it in the return envelope. They should pay for the return envelope if not then they really are cheap but there is not much you can do now but throw everything away. Lets assume they pay the postage though.
Put all the papers that came with it in this envelope and send it back to them. Now you wont have to worry about using up your garbage space. This may seem mean but remember they sent you junk to have to deal with you never asked for and they are not paying as much for postage as you because they pay so much in postage they get a deal on it.
If you do not fill this is mean there are ways to improve on this. Of course you can add more stuff to your envelope like more paper, or even add in as many items as you can stuff in the envelope and as heavy as you can. The heavier it is the more they pay and the more they have to throw away themselves.
If you are feeling particular bitter and I must admit once in awhile this would be fair you can cut out the front of the return envelope and tape or past it onto an large envelope or even something else like a trash bag but I would suggest something you can easily fit in a mailbox because if you took a trash bag like that to the mailbox they might get wise and not let you send it and I am not 100% sure they will send it this way but I am fairly certain they will.
You can always use a box and drop it in the mailbox slot on the inside the post office will even give you free boxes and the company already was nice enough to send you the postage.
If enough people silently strike against the big companies then maybe they will stop sending us so much junk and wasting so many resources to cut the trees and gas to send the mail to you, etc. Viva La Revolution!